The present invention relates to a discharge lamp and a lamp unit. In particular, a discharge lamp and a lamp unit used as a light source for an image projection apparatus such as a liquid crystal projector and a digital micromirror device (DMD) projector.
In recent years, an image projection apparatus such as a liquid crystal projector and a DMD projector has been widely used as a system for realizing large-scale screen images, and a high-pressure discharge lamp having a high intensity has been commonly and widely used in such an image projection apparatus. In the image projection apparatus, light is required to be concentrated on a very small area of a liquid crystal panel or the like, so that in addition to high intensity, it is also necessary to achieve nearly a point light source. Therefore, among high-pressure discharge lamps, a short arc type ultra high pressure mercury lamp that is nearly a point light and has a high intensity has been noted widely as a promising light source.
Referring to FIGS. 21A to 21C, a conventional short arc type ultra high pressure mercury lamp 1000 will be described.
FIG. 21A is a schematic top view of a lamp 1000. FIG. 21B is a schematic side view of a lamp 1000. FIG. 21C is a cross-sectional view taken along line c–c′ of FIG. 21A.
The lamp 1000 includes a substantially spherical luminous bulb 110 made of quartz glass, and a pair of sealing portions 120 and 120′ (seal portions) made of also quartz glass and connected to the luminous bulb 110. A discharge space 115 is inside the luminous bulb 110. A mercury 118 in an amount of the enclosed mercury of, for example, 150 to 250 mg/cm3 as a luminous material, a rare gas (e.g., argon with several tens kPa) and a small amount of halogen are-enclosed in the discharge space 115.
A pair of tungsten electrodes (W electrode) 112 and 112′ are opposed with a certain gap in the discharge space 115, and a coil 114 is wound around the end of the electrode 112 (or 112′). An electrode axis 116 of the electrode 112 is welded to a molybdenum foil (Mo foil) 124 in the sealing portion 120, and the W electrode 112 and the Mo foil 124 are electrically connected by a welded portion 117 where the electrode axis 116 and the Mo foil 124 are welded.
The sealing portion 120 includes a glass portion 122 extended from the luminous bulb 110 and the Mo foil 124. The glass portion 122 and the Mo foil 124 are attached tightly so that the airtightness in the discharge space 115 in the luminous bulb 110 is maintained. The principle on the reason why the luminous bulb 110 can be sealed by the sealing portion 120 will be briefly described below.
Since the thermal expansion coefficient of the quartz glass constituting the glass portion 122 is different from that of the molybdenum constituting the Mo foil 124, the glass portion 122 and the Mo foil 124 are not integrated. However, by plastically deforming the Mo foil 124, the gap between the Mo foil 124 and the glass portion 122 can be filled. Thus, the Mo foil 124 and the glass portion 122 are pressed and attached to each other, and the luminous bulb 110 can be sealed with the sealing portion 120. In other words, the sealing portion 120 is sealed by attaching the Mo foil 124 and the glass portion 122 tightly for foil-sealing.
The Mo foils 124 of the sealing portions 120 and 120′ have the same size and a rectangular plane shape, and are positioned at the center of the internal portion of the respective sealing portions 120 and 120′ so that the directions x (width directions) perpendicular to the thickness directions Z of the foils are in the same direction. In other words, the pair of the sealing portions 120 and 120′ is coupled to the ends of the luminous bulb 110 so that the flat Mo foils 124 are symmetrical with respect to the luminous bulb 110 as the center.
The Mo foil 124 includes an external lead (Mo rod) 130 made of molybdenum on the side opposite to the side on which the welded portion 117 is positioned. The Mo foil 124 and the external lead 130 are welded with each other so that the Mo foil 124 and the external lead 130 are electrically connected at a welded portion 132. The external lead is electrically connected to a member (not shown) positioned in the periphery of the lamp 1000.
Next, the operational principle of the lamp 1000 will be described. When a start voltage is applied to the W electrodes 112 and 112′ via the external leads 130 and the Mo foils 124, discharge of argon (Ar) occurs. Then, this discharge raises the temperature in the discharge space 115 of the luminous bulb 110, and thus the mercury 118 is heated and evaporated. Thereafter, mercury atoms are excited and become luminous in the arc center between the W electrodes 112 and 112′. As the pressure of the mercury vapor of the lamp 1000 is higher, the emission efficiency is higher, so that the higher pressure of the mercury vapor is suitable as a light source for an image projection apparatus. However, in view of the physical strength against pressure of the luminous bulb 110, the lamp 1000 is used at a mercury vapor pressure of 15 to 25 MPa.
As a result of in-depth research, the inventors of the present invention found that the lifetime of the conventional lamp 1000 is shortened by leaks occurring in the sealing portions 120. More specifically, the sealing portions 120 of the lamp 1000 are sealed by attaching the Mo foils 124 and the glass portions 122 tightly, so that as shown in FIGS. 22A and 22B, an internal stress 40 occurs in the direction perpendicular to the surface of the foil (the Z direction in FIGS. 22A and 22B) on the Mo foil 124. Therefore, when the glass portions 122 are deteriorated with use of the lamp 1000 and the strength of the glass portions 112 is reduced, the glass portions 112 can be split by the internal stress 40 on the Mo foils 124 at a certain point. When the glass portions are split, air is let into the sealing portions 120 so that the Mo foils 124 are oxidized. Thus, the conductivity of the Mo foils 124 is lost, so that the lamp 1000 stops its operation.
Furthermore, in the welded portions 132 in the sealing portions 120, the Mo foils 124 and the external leads 130 are substantially in point contact with each other, so that the contact area therebetween is small. Therefore, a local increase in the temperature is often caused by current flowing from the external leads 130 to the Mo foils 124. Molybdenum constituting the Mo foils 124 has the nature that it is oxidized at 350° C. or more, so that this local increase in the temperature causes a large problem when the Mo foils 124 are used. There may be an approach of suppressing the local increase in the temperature of the welded portion 132 by increasing the size of the Mo foils 124 to increase the heat capacity. However, it is difficult to adopt this approach in the context that there is a great demand for compactness of the lamp size with a trend of compactness of image projection apparatuses. Furthermore, to achieve high intensity, there is a tendency of reducing the electrode distance L between the W electrodes 112 and 112′ (to achieve a short arc) to allow a large amount of current to flow. Therefore, the problem of the local increase in the temperature of the welded portions 132 may become more serious. Furthermore, even if the oxidation of the Mo foils 124 does not occur, the local increase in the temperature of the welded portions 132 may generate a starting point of cracks in the glass in the periphery of the welded portions 132. Therefore, the temperature increase is problematic also in view of a cause of leaks of the sealing portions 120.